1. Field of the Invention
The invention is based on a piston pump for delivering a fluid, which is used in particular in brake systems of vehicles.
2. Description of the Prior Art
Such piston pumps are used in vehicles with hydraulic or electrohydraulic vehicle brake systems, preferably as return feed pumps, in order to raise or lower a brake pressure in the wheel brake cylinders selectively, as a result of which the brake pressure in the wheel brake cylinders can be regulated. Such regulation can be performed for instance in an anti-lock brake system (ABS), a traction control system (TC system), an electronic stability program system, and so forth. FIGS. 1 through 3 show a conventional piston pump, which is used in a vehicle brake system. As can be seen from FIGS. 1 through 3, a conventional piston pump 1 includes a piston assembly 2 that has a first piston element 2.1, having a sealing element 13; a second piston element 2.2; an inlet valve 5; an outlet valve 6; and a cylinder 8. The inlet valve 5 is embodied as a check valve and includes a cage element 11, in which an inlet valve spring 5.2 and an inlet valve sealing element 5.3 are disposed; the inlet valve sealing element 5.3 is embodied for instance as a sealing disk, which can cooperate sealingly with a corresponding inlet valve seat 5.1 that is disposed on the second piston element 2.2, and the second piston element 2.2 is connected to the cage element 11 by nonpositive engagement. The outlet valve 6 is likewise embodied as a spring-loaded check valve and is disposed in a cap element 12. The outlet valve 6 is opened when a pressure in a compression chamber 8.1 is greater than a spring force of an outlet valve spring 6.3, which force acts on an outlet valve sealing element 6.2 of the outlet valve 6, and as a result, the outlet valve sealing element 6.2 is pressed out of an outlet valve seat 6.1 disposed at an outlet opening 8.3 of the cylinder 8.
During an intake stroke of the piston assembly 2, fluid is aspirated radially via a filter sleeve 9 and transverse bores 3 disposed in the first piston element 2.1 and is carried, via the longitudinal bore 4, corresponding to the transverse bores 3, in the second piston element 2.2 and via the opened inlet valve 5 into the compression chamber 8.1, which is disposed in the cylinder 8 between the inlet valve 5 and the outlet valve 6. Once top dead center is reached, the direction of motion of the piston group 2 is reversed, so that the second piston element 2.2 having the inlet valve seat 5.1 is pressed sealingly against the inlet valve sealing element 5.3, via the first piston element 2.1 driven by an eccentric element 14 disposed in an eccentric chamber 15, and the inlet valve 5 is closed. Now, a pressure buildup takes place in the compression chamber 8.1 until such time as the pressure in the compression chamber 8.1 is greater than the spring force of the outlet valve 6, as a result of which the fluid that is under pressure is conducted via an outlet opening 8.3 and the opened outlet valve 6 from the compression chamber 8.1 into an outlet line, not shown.
Once bottom dead center is reached, the direction of motion of the piston assembly 2 reverses again, so that the outlet valve 6 closes again, and the intake stroke begins again; a restoring force F2 of a restoring spring 10, which is disposed in the compression chamber 8.1 and guided by a cylindrical wall 8.4 and which is embodied for instance as a spiral spring with polished end turns and which is braced on a cylinder bottom 8.2 and on the cage element 11, acts against the cage element 11 of the inlet valve 5 and thus against the second piston element 2.2 in order to move the piston assembly 2 back in the direction of top dead center again. Axial forces F1 and F2, which are respectively effected by the eccentric element 14 and introduced via the first piston element 2.1 and by the restoring spring 10 and introduced via the cage element 11, and radial forces F3, which are generated by the prevailing system pressure, act during operation on the second piston element 2.2 that bears the inlet valve seat 5.1. The second piston element 2.2 embodied as a valve seat must be produced as a stable component, preferably as a metal component, and can be weakened only conditionally by intake bores in the form of transverse bores. The transverse bores 3 are therefore made in the first piston element 2.1, resulting in a relatively long inlet region embodied as the longitudinal bore 4.